1. Field of the Invention
The present invention generally relates to an optical communication system, and more particularly to an amplifier circuit suitably used to amplify an input current obtained by converting a light signal.
In an optical communication system, information is transmitted by light, while in many cases the necessary amplifying and switching is performed by amplifying and switching an electric signal. For example, a light signal transmitted over an optical fiber cable is converted into an electric signal by means of a photodiode. The electric signal is then amplified and is subjected to a predetermined process. The amplifying of the electric signal is carried out by a first amplifier circuit (preamplifier) that converts a current signal output by the photodiode into a voltage signal, and a second amplifier circuit (main amplifier circuit) that amplifies the voltage signal.
The present invention relates to an amplifier circuit which converts a current signal into a voltage signal in an optical communication system.
2. Description of the Related Art
Generally, each subscriber terminal is connected to a switching office by an optical fiber cable. In the switching office, a received light signal is converted into an electric signal, which is then subjected to a predetermined signal process. The light signal transmitted over the optical fiber and received at the switching office has a variation in the receive level. Such a variation in the receive level is due to, for example, the distance between the switching office and each subscriber terminal or the differences among the output levels of the light signals transmitted from the subscriber terminals. When the levels of the received light signals are varied, current signals obtained by converting the received light signals are correspondingly varied. A preamplifier circuit, which converts the current signal (input current) into a voltage signal (output voltage), has a certain tolerance of variation in the input current. However, if there is a large variation in the level of the received light signal, the preamplifier will be saturated and the duty ratio of the output voltage will become different from that of the input signal. This will cause an error in a subsequent signal process carried out at a following stage, such as a decision-making process (discrimination process).
FIG. 1 is a circuit diagram of a conventional preamplifier circuit, which is made up of bipolar transistors Q.sub.1 and Q.sub.2, a load resistor RL, a feedback resistor Rf and an output resistor R. The load resistor RL has an end connected to a constant-voltage source V.sub.CONT. A photodiode PD having a cathode connected to another constant-voltage source V.sub.CONT' converts the light input into a current input. Since the transistor Q.sub.1 has a high base impedance, most of the input current passes through the feedback resistor Rf and flows to ground through the output resistor R. Hence, the emitter potential of the output transistor (which may be called a buffer transistor) Q.sub.2 is decreased. When no light input is received, no current flows in the feedback resistor Rf. Hence, the emitter potential of the transistor Q.sub.2 is increased.
However, the preamplifier shown in FIG. 1 has the following disadvantages, which will be described with reference to FIG. 2. FIG. 2 shows a dc input/output response characteristic of the preamplifier shown in FIG. 1. As shown in FIG. 2, if the input current has an excessive amplitude waveform with respect to the input/output characteristic (in other words, if a large current input takes place), the collector potential of the transistor Q1 shown in FIG. 1 is decreased, and a sufficient base-collector voltage cannot be obtained. Hence, the circuit is saturated. Thus, as shown in FIG. 2, the duty ratio of the output voltage waveform (output response waveform) is degraded. A degradation of the above duty ratio may cause an erroneous decision operation for each bit in a decision making circuit coupled to the preamplifier through the main amplifier. The decision making circuit determines, for each bit, whether the signal is either "0" or "1".